clock. This means that a new access time slot must be decided
upon, because which ever half of the 1 Mhz clock is used, it
will always interfere with the load clock. The new access slot
must be between load clock pulses, but not too close to the
start of a load clock pulse so as to interfere with video access
timing to the RAM. Figure 2 shows the new access signal, this is
produced by exclusive ORing the 1 and 2 Mhz then inverting the
resultant signal. The wait gate is produced as before and the
phases of the 2 and 4 Mhz clocks altered to suit this using the
theory previously described. Again all the waveforms are shown
in figure 2 so that they may be compared to the original clocks
Figure 3 shows the circuit diagram, this is fairly straight
forward and should present few problems to the constructor. The
final connection to the Nascom is somewhat more involved but,
providing that the instructions are followed carefully, there
should be no real problems
In addition to the 4 integrated circuits used, IC 49 must be
relocated on the Snowdinger 2 board. The socket for IC 49 must
be of the wire-wrap type, with pins 2 and 6 cut short, This is
so that the whole board can plug into Nascoms socket for IC 49,
with the new 2 and 4 Mhz clocks connected, via wires, to pins 6
and 2 respectively. Any connections made to IC 49 should then be
made to the wire wrap socket. The prototype used wire wrap
sockets for all the connections but any method of construction
may be used. Follow the circuit diagram carefully and check all
your work as you go along.
The board should have 6 wires for connection to the main
board of the Nascom. In the circuit diagram of figure 3 these
are marked 1 to 6. Using the following list make all your
Remove IC 58 completely from the main board.
Remove IC 49 and place it on the Snowdinger 2, taking care
to have it correctly orientated.
Remove IC 71, bend pins 2 and 4 outwards and then replace
Remove IC 69, bend pin 3 outwards and then replace it.
Take wire 1 and push it into pin 2 of IC 49’s socket.
Take wire 2 and push it into pin 6 of IC 49’s socket.
Solder wire 3 to pin 3 of IC 69.
Take wire 4 and push it into pin 9 of IC 58’s socket.
Solder wire 5 to the processor end of R9 on the main